U.S. Pat. No. 5,106,270 to Goettel et al discloses an air compressor comprised of two low pressure cylinders each of which discharges low pressure air into respective intercoolers to cool the compressed air before it enters a common manifold connection and inlet flange of a high pressure cylinder. A single intercooler core design is also available that collectively receives the air discharged from such low pressure cylinder heads and cools the air before entering the high pressure head's inlet flange for the second stage of compression.
The compressor can be driven by an electric motor, as disclosed in the Goettel et al patent, though in times past, compressors in locomotives were driven directly by the diesel engine of the locomotive. In this manner, while the diesel engine was idling, the compressor continued to run, though at the slower idle speed of the diesel engine.
More recent compressor designs, however, are operated by electric motors in a stop/start fashion. In this mode of operation, the compressors are started when pressurized air is needed and stopped when pressurized air is not needed. Such electric motors operate from a voltage generated by an alternator, disposed in the locomotive, which is driven by the diesel engine of the locomotive. When diesel engine RPM is low, such as in an idle condition, the alternator produces only a limited amount of electrical power. Such a limited amount of power may be insufficient to operate the compressor motor at a speed sufficient for the compressor to deliver the required amount of compressed air to the train. When this occurs, the air compressor needs to operate at a speed greater than that at which the motor is capable of when it is only supplied by the electrical characteristics of the alternator.
For this reason, compressor motors may have a dual pole, dual speed configuration. For example, the motor may consist of the same number of magnetic poles as the supply voltage alternator. For low speed operation, since the poles of the motor and alternator are equal, the compressor turns at essentially the same speed as the alternator (and the mechanical drive of the diesel engine) less any losses, of course.
If the compressor can run faster than engine speed (such as an idle speed), to assure a compressed air output to overcome train line losses there will be only the need to reduce the number of active motor poles. For example, if the number of motor poles is reduced in half the compressor will run at twice the diesel engine/alternator speed. In this manner, the locomotive crew can operate the locomotive at a lower engine speed (to save fuel and reduce engine wear) while, at the same time, produce a sufficient amount of compressed air for the brakes and other pneumatically operated devices.
When additional air pressure is called for, the compressor motor is signaled to operate at the higher speed. When this occurs, the compressor is unloaded (exhausted) of air pressure so that the motor can start (transition) under unloaded conditions. When the compressor is unloaded, the compressor rotates freely and thus places a very light load on the electric motor. If the motor is required to start or transition against a pressure load in the compressor, the rotor of the compressor can appear to the motor to be locked, and can thereby burn out the motor, as the motor draws large amounts of current to overcome the force of compression in the compressor.
The compressor rotor includes a crankshaft that operates the pistons disposed in the cylinders of the compressor. The pistons being the mechanism by which the compressed air is formed in the compressor. It is therefore understandable that with air pressure in the cylinders acting against the pistons and thus against the crankshaft of the compressor, the electric motor connected to drive the compressor has a difficult task in rotating the crankshaft.
The air compressor will normally unload when the increase in the main reservoir pressure reaches about 140 psig. At this point, a compressor governor or compressor control switch admits air to an unloader line connected to unloader inlet valves located on the cylinder heads to move and hold an inlet valve off its seat thereby preventing further compression of air. The cylinders, cylinder heads and intercooler are vented to atmosphere via an exhaust vent in the unloader valve. The intercooler pressure vents to atmosphere through the unloader valves and vents. Such unloader venting takes about 25 seconds.
Historically, this time period was not important because the compressor was operated constantly by the diesel engine of the locomotive and would load and unload as needed (under the control of the governor). The time it takes for a dual pole configuration motor to transition from its relatively slow speed (twelve pole) operation to the doubling high speed (six pole) operation is on the order of two to three seconds. Hence, when the motor changes speed there may still be air pressure in the high pressure head of the compressor, as supplied by the intercooler(s). It is therefore important that intercooler air pressure be discharged quickly so that the compressor motor does not have to start, i.e., change speeds, against a pressure load in the high pressure cylinder of the compressor.